1. Field of the Invention
This invention relates generally to the field of integrated circuits fabrication, and more particularly, to a process of forming local field oxide isolation regions without bird's beak.
2. Description of the Prior Art
In the conventional integrated circuits fabrication, isolation is achieved by either local oxidation (LOCOS) or shallow trench isolation (STI). However, both are suffered from the so-called bird's beak formation which is caused by oxygen encroachment into the active regions during the oxidation. Referring now to FIG. 1, it illustrates a typical bird's beak region 14 at the areas where the field oxide 12 is adjacent to the active regions of the silicon substrate 10. These bird's beak regions cause problems in subsequent processing particularly as the allowable dimensional tolerances of the integrated circuits become smaller.
Within the prior art, there are some references addressing the bird's beak issue, however, none appears to be directed to the specific application of the present invention, namely the use of rough oxide mask to form multiple trenches for smoother field oxide growth.
For example, U.S. Pat. No. 5,374,583 to Lur et al. (the entire disclosure of which is herein incorporated by reference) discloses the use of aluminum--silicon alloy mask to etch a set of narrow trenches. Another paper titled "Nano-Trenched Local Oxidation of Silicon Isolation Using Island Polysilicon Grains" by Kwon et al., J. Electrochem. Soc., Vol. 143, No. 2, pp. 639-642, 1996, teaches the use of island polysilicon grains (IPG) mask. But the processes are too complicated and the materials are not often used in a semiconductor factory to be practically employed for integrated circuits mass-production.